1. Field of the Invention
The present invention relates to an improved sizing composition and method for its preparation.
2. Description of the Prior Art
In the papermaking process, sizing agents are generally included to make the finished paper product resistant to liquid penetration. This is generally accomplished by two alternative approaches. In the first approach, the sizing agent can be added to the papermaking process pulp slurry and is referred to as an "internal sizing agent." In the second approach, the sizing agent can be applied as a coating to the surface of the paper after it has been formed and partially dried.
In internal sizing, the sizing agent is dispersed through the pulp fibers before sheet formation. In this manner, a uniform distribution of the sizing agent exists throughout the fiber matrix of the resulting paper product.
One of the most widely used and effective sizing agents is rosin. Rosin is a complex mixture of about 90% resin acids and about 10% neutral matter. Rosin is insoluble in water in the free or uncombined state.
Rosin size is generally made by neutralizing or saponifying the rosin with one or both of the common alkalis, potassium or sodium hydroxide. The potassium and sodium salts of rosin are readily dispersed in water and concentrated solutions of these materials provide useful sizing agents for wood pulp that is used in paper making.
Several U.S. patents disclose that rosin based sizing compositions are improved with the addition of a reaction product of urea and an acid. The reaction product of urea and an acid increases the total acidity of the resulting sizing compositions. The increased total acidity, which is greater than that of the rosin size alone is believed responsible for the superiority of these compositions over those of the prior art.
U.S. Pat. Nos. 4,022,634 and 4,093,779 to Emerson et al disclose the reaction product of urea with acids such as sulfamic, phosphoric, oxalic, methane-sulfonic, trichloracetic, nitric, sulfuric, hydrochloric, stearic, and acetic. U.S. Pat. No. 4,025,354 to Emerson et al focuses upon the urea-sulfamic acid reaction. U.S. Pat. No. 4,141,750 to Emerson et al and U.S. Pat. No. 4,483,744 to Emerson discloses the reaction product of urea and at least one Lewis acid from the group including sulfuryl chloride, chloro-sulfonic acid, thionyl chloride, benzenesulfonyl chloride, benzenesulfonic acid, ortho-toluenesulfonic acid, para-toluenesulfonic acid, ortho-toluenesulfonyl chloride and para-toluenesulfonyl chloride. U.S. Pat. No. 4,437,894 to Emerson discloses the reaction product of urea and formic acid.
Also of interest is U.S. Pat. No. 2,665,983 to Bakalar et al which discloses sizing agents composed of hydrocarbons in the presence of a polyphosphoric acid compound, preferably together with a mutual solvent for the polyphosphoric acid compound and the sizing agent used. U.S. Pat. No. 2,771,464 to Hastings et al. discloses the use of fortifying agents useful for enhancing the effect of rosin size in the manufacture of sized paper. The fortifying agent is comprised of the reaction product of rosin, maleic anhydride, and dehydrated citric acid. U.S. Pat. No. 1,904,251 to Rafton discloses the use of citric acid or other soluble citrates such as sodium, potassium or ammonium citrate as precipitation inhibitors for aluminum compounds, such as aluminum hydroxide or hydrate for the purpose of preventing sticking of the stock on the paper machine.
In internal sizing with rosin, co-addition of aluminum sulfate is necessary to produce the sizing effect. The aluminum ions react with rosin to form aluminum rosinate, a cationic hydrophobic material that absorbs onto the oppositely charged cellulose fibers. Aluminum ions can also be furnished by other equivalent compounds such as aluminum chloride and sodium aluminate.
To facilitate the dispersion of the sizing agent and enhance the sizing effect, it is desirable to dilute the sizing solutions with water before introduction to the paper-making process. Unfortunately, the reaction of rosin with polyvalent cations is not limited to aluminum. Calcium and magnesium ions, two common constituents of hard water, also compete with aluminum ions and react with rosin to produce insoluble salts. The insoluble calcium and magnesium salts are virtually ineffective as sizing agents and render the rosin acids inactive to aluminum ions. Thus, the competition of the calcium and magnesium ions with aluminum ions for rosin acids results in a decrease in available rosin and reduced sizing effectiveness.
In addition, the use of hard water for diluting rosin size can render the sizing agent prone to precipitation. In mild cases this tendency to precipitate is limited to the formation of turbid size "teas" or dilutions. Generally, if turnover of the turbid material is rapid, other more serious effects can be minimized. However, if the turbid material in the size composition is allowed to settle, or if excessively hard water is used, insoluble calcium and magnesium rosinate can precipitate and form deposits in storage tanks and feed lines which can obstruct the delivery of the size system to paper machines. This results in loss of sizing in the paper produced. Thus, a sizing agent that is resistant to precipitation in hard water, is desirable